1. Field of the Invention
The present invention relates to an apparatus and a method for transmitting uplink feedback data in a broadband wireless mobile communication system. More particularly, the present invention relates to an apparatus and a method for transmitting uplink feedback data via a feedback channel configured by using a preferred channel selected by a terminal through a downlink channel when feedback information should be transmitted non-periodically, in a Time Division Duplexing (TDD) Orthogonal Frequency Division Multiplexing (OFDM) broadband wireless mobile communication system.
2. Description of the Related Art
In a wireless mobile communication system, channel quality of a reception signal may deteriorate over time. Therefore, an Automatic Repeat reQuest (ARQ) technique is applied to ensure that communication signals can be correctly received. In particular, a recent mobile communication system introduces a Hybrid Automatic Repeat reQuest (HARQ) technique for more accurate communication of transmission signals. Under the HARQ technique, feedback information including Acknowledgement (ACK) and Negative Acknowledgement (NAK) is transmitted through an additional feedback channel. Furthermore, in a recent cellular wireless mobile communication system, to more efficiently utilize wireless resources, signals are transmitted by adaptively changing a modulation method and a coding rate according to a channel change scheme, which is called an Adaptive Modulation and Coding (AMC) scheme. To apply the AMC scheme, a transmitter needs to have channel information in advance. In a general Time Division Duplexing (TDD) system, the transmitter predicts a channel state based on a reciprocity of uplink/downlink channels or identifies the channel state through information transmitted via an additional feedback channel. In a cellular wireless mobile communication system, the effects of interference signals may differ in the uplink/downlink channels. Hence, in most mobile communication systems, including an IEEE 802.16e system, an additional feedback channel (for example, Channel Quality Indication (hereinafter, referred to as CQI)) is established and a terminal transmits downlink channel information, for example, Carrier to Interference and Noise Ratio (hereinafter, referred to as CINR) to a base station via this feedback channel.
Meanwhile, to inform of the ACK, NAK and CINR, the system sets a portion of the uplink wireless resource as a feedback channel, leading to a reduction in the amount of usable wireless resource. Therefore, the feedback channel needs to be limited to a reasonable level in consideration of the number of users in the system, to reduce waste due to overhead. As described above, to efficiently support an uplink feedback channel with limited wireless resources, a proper technique for a feedback channel design and an efficient method for resource management are required.
In a conventional IEEE 802.16e system, a part of the uplink frame is designated as a control channel, and a portion of the designated part of the uplink frame is used as the ACK, NAK and CQI channels. The system allocates a specific sub-channel to a terminal requiring an uplink feedback channel and reallocates the sub-channel if the terminal does not need the uplink feedback channel. In the conventional method, if a specific sub-channel is selected, N fixed tiles are selected, and a terminal transmits feedback information via the sub-channel comprised of corresponding tiles. For example, the IEEE 802.16e system defines three symbol sections in the uplink as a control channel to have 35 sub-channels with a 4×3 tile structure, and uses the sub-channels as the ranging, ACK, NAK and CQI channels. In the conventional method as described above, when the sub-channel is allocated to each terminal for the ACK, NAK and CQI channel feedback, the channel state of each terminal is not considered. Therefore, the conventional method has a disadvantage in terms of efficiency in resource allocation. For example, a base station may allocate a sub-channel comprised of preset number of tiles to a corresponding terminal as a feedback channel regardless of the channel state of the terminal, and each terminal uses the sub-channel allocated by the base station.
In contrast, a terminal may select a preferred tile in consideration of the channel state measured from a downlink. In this case, to inform the base station of information on the tile selection, the terminal needs additional signaling. There are two methods for informing the base station of information on the selected tile. The first method is a method of directly transmitting corresponding information as a message. The second method is a method of indirectly transmitting the information by multiplying each tile by a user-identification code. As described above, the method of allocating the feedback sub-channel by selecting a preferred tile by a terminal has an advantage of reducing overhead according to the allocation and reallocation of a resource. That is, the feedback sub-channel allocation method applied to the conventional IEEE 802.16e system is appropriate for a periodic transmission because when informing of the CINR via the CQI channel, the CINR is informed at a constant interval regardless of any changes in the channel. However, this is disadvantageous in that secondary overhead may occur due to frequent allocation and reallocation processes in a non-periodical transmission. In contrast, in the method of allocating the feedback sub-channel by selecting a preferred tile by a terminal, the terminal informs the base station of the CINR non-periodically only when some changes occur, thereby reducing the usage rate of the uplink feedback channel and consequently decreasing overhead.
Meanwhile, in the method of allocating the feedback sub-channel by selecting a preferred tile by the terminal, the base station needs information on the tile selected by the terminal in advance to receive the feedback information of each terminal. For example, when the number of tiles that can be used is T and the number of the tiles selected by the terminal is N (T>N), the number of combinations that the terminal can select is TCN. That is, since the base station does not have information on which tile was selected by the terminal, it should perform detection for all combinations. Moreover, if the number of terminals using the uplink feedback channel is increased, the amount of calculations performed by the base station is also increased in proportion thereto. Accordingly, a method is desired which can achieve a compromise in terms of the increase of resource usage efficiency and the amount of calculations needed to be performed by the base station.